docs: def _periodic_callback(self):
          """ Will be started on first emit """
          try:
              self.notify(self._state)  # emit to all subscribers
          except Exception:  # pylint: disable=broad-except
              self._error_callback(*sys.exc_info())
  
          if self._subscriptions:
              # if there are still subscriptions register next _periodic callback
              self._call_later_handle = \
                  self._loop.call_later(self._interval, self._periodic_callback)
          else:
              self._state = NONE
              self._call_later_handle = None